[GHC] #15648: Core Lint error with source-level unboxed equality

[GHC]

#15648: Core Lint error with source-level unboxed equality
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15209            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I worry a malicious user of TH might get around the `checkValidType`
 check.

 Why do we need

 * If `x :: t1 ~# t2`, then `x` is a `CoVar`

 ? We clearly need the other direction of implication (that all `CoVar`s
 have coercion types), but I don't see the harm in allowing non-`CoVar`s to
 have coercion types. They'd be useless, but also harmless, I think.

 I wonder if there is just some code that looks at an `Id`'s type to
 determine whether it's a coercion variable instead of checking `isCoVar`.
 This might be from the fact that `CoVar`'s distinguished nature in
 `IdDetails` is a relatively new innovation. It used to be that looking at
 the type was the only way to tell coercion variables from others.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15648#comment:7>
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